The present invention relates generally to superconducting magnets, and more particularly to an assembly for venting cryogenic gases away from a superconducting magnet when gas pressure builds to a potentially dangerous level due to magnet quench or reversion to a non-superconductive state.
It is well known that magnets can be made superconductive by enclosing them within a pressure vessel, or cryostat, which contains a reservoir of liquid helium or some other cryogenic liquid. The extreme cold reduces the resistance in the coils of the magnet such that when a power source is initially connected to the coil to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (“MRI”).
During normal superconducting operation of the magnet, the cryostat must be a sealed or closed system for reasons including, but not limited to: preventing leakage of helium gas or other cryogenic gas from the cryostat; preventing depletion of liquid helium or other cryogenic liquid from the reservoir; and preventing the ingress of air which will freeze at the low temperatures inside the vessel. In the event of an undesired magnet quench, a reversion of the magnet to a non-superconductive state, or a loss of the insulating vacuum in the cryostat, a rapid and potentially dangerous helium gas high pressure buildup in the cryostat will occur. Such a pressure buildup requires pressure relief through rapid venting of the gas to the atmosphere outside the superconducting magnet. Referring to FIG. 1, a replaceable burst disc 10 may be interposed within a magnet vent assembly 20, which disc 10 is designed to rupture at a predetermined pressure thereby opening the cryostat to an atmospheric vent.
The burst disc may require replacement for a variety of reasons, including, but not limited to: a burst disk rupture, damage due to a liquid helium transfer, or preventative maintenance. Ideally a burst disc would be replaced when the magnet is still energized, as considerable time and expense is involved in de-energisation. A service engineer must replace the burst disc. If the magnet is energized and a magnet quench occurs during the replacement of the burst disc, the service engineer would be exposed to cold helium gas in the confined space of an MRI imaging room, or in a vehicle compartment of a portable MRI system. This gas will displace oxygen from the room giving a risk of asphyxiation and may cause cold burns. Therefore, a service engineer may be required to wear bulky protective gear to protect against exposure to cryogenic gas.
It is desirable to replace a burst disc while the magnet is energized. It is also desirable to prevent the possible release of a cryogenic gas in an MRI imaging room or a vehicle compartment of a portable MRI system during a replacement of a burst disc.